The present invention relates to an A/F (air-fuel) ratio sensor used for detecting the A/F ratio of an air/fuel mixture supplied to a combustor such as an internal combustion engine. More particularly, the present invention relates to an A/F ratio sensor that employs an oxygen pump element composed of an oxygen ion-conductive solid electrolyte and an oxygen gas sensing element composed of an electron-conductive semiconductor metal oxide.
With a view to improving fuel economy and reducing emissions while providing improved operability, it has been proposed that feed-back control be performed on combustors, such as internal combustion engines, so as to attain a desired A/F ratio according to specific operating conditions. Numerous A/F ratio sensors have been proposed for use in such feedback control.
For example, according to the system shown in Unexamined Published Japanese Patent Application No. 190652/1984, an oxygen concentration electrochemical cell element employing atmospheric air as a reference oxygen source and an oxygen pump element using atmospheric air as an oxygen supply source are placed in a face-to-face relationship with an enclosed compartment formed therebetween. The compartment communicates with the ambient atmosphere by way of a diffusion-limiting portion. The oxygen pump element is controlled so that the oxygen partial pressure in the enclosed compartment is held constant, with the electric current (pump current) that flows through the pump element being used to indicate the A/F ratio of the sensed atmosphere over the full operating range of the combustor, including both the fuel-lean region and fuel-rich region.
Another A/F ratio sensor system is described in Unexamined Published Japanese Patent Application No. 153155/1983. In this system, an oxygen concentration electrochemical cell element which contacts the sensed atmosphere on one side and an enclosed compartment on the other side and an oxygen pump element which also contacts the sensed atmosphere on one side and the enclosed compartment on the other side are placed in a face-to-face relationship with the enclosed compartment being formed therebetween. The compartment communicates with the sensed atmosphere by way of a diffusion-limiting portion. The oxygen pump element is controlled so that the oxygen partial pressure in the enclosed compartment is held constant, with the pump current being used to indicate the A/F ratio of the ambient atmosphere over the full operating range including both the fuel-lean region and the fuel-rich region.
The first type of A/F ratio sensor described above requires a channel for introducing atmospheric air, hence cannot be sealed completely, and may be adversely affected during service by foreign matter such as moisture. The second type of A/F ratio sensor mentioned above does not require the introduction of atmospheric air, but its output characteristics are ambiguous in that one output value is associated with two different values of A/F ratio, specifically, values in both the fuel-lean and fuel-rich regions. In order to ensure a nonambiguous, one-to-one correspondence between the output value and sensed A/F ratio over the full operating range including both the fuel-rich and fuel-lean regions, an additional sensor is required to indicate whether the combustor is operating in the fuel-rich region (.lambda.&lt;1) or fuel-lean region (.lambda.&gt;1).
This problem may be overcome by an A/F ratio sensor in which an oxygen gas sensing element that employs an electron-conductive semiconductor metal oxide such as TiO.sub.2 and which does not require a reference source is combined with an oxygen pump element composed of an oxygen ion-conductive solid electrolyte. However, an oxygen gas sensing element made of an electron-conductive semiconductor metal oxide such as TiO.sub.2 and an oxygen pump element made of a ZrO.sub.2 -based solid solution compound are difficult to produce simultaneously by conventional techniques since they require different temperatures and atmospheres for firing. On the other hand, if the two elements are fired separately and joined together afterward, it is very difficult to attain a perfectly fitting unitary assembly.